Whenever the challenge rules say it's over. :P When making challenges and having to set a completion point though, I usually have it as Champion Rival (RBY/FRLG), Red (GSC/HGSS), Steven (RSE), Cynthia (DPPt), Ghetsis (BW), and Iris (B2W2). If I'm doing a regular playthrough, first round E4 is always the end for me.